The invention relates to components for the boiler area of power stations or refuse incineration plants inclusive of special waste incineration plants, as well as such plants in which corrosive, abrasive loading or the like can lead to premature damage.
Since combustion equipment such as power stations or refuse incineration plants fulfill the purpose always only in conjunction with upstream or downstream equipment such as conveyors, heat exploitation and flue gas washing installations, the operational readiness of all individual components is decisive for overall availability. In the case of combustion of solid fuels or solid industrial or special wastes as well as domestic refuse, plants of that kind usually consist of delivery systems for the solid fuels, a furnace in the form of a drum furnace or shaft furnace, optionally with an afterburning chamber, a steam boiler for waste heat utilization, a flue gas washer and the chimney. The combustion temperatures in the furnace lie between about 800 to 1200xc2x0 C. for a dwell time of up to one hour for the combustion material. The arising flue gases have to be cooled from the combustion chamber by way of the boiler flues towards the dust extraction installations, wherein the continuous temperature difference can be up to 800xc2x0 C. Since flue gas as raw gas can contain up to more than 10,000 mg/m3 of dust in the raw gas and always still up to 100 mg/m3 of dust in the clean gas, a deposit of these flue gas constituents at the water-conducting or steam-conducting heat carrier system, such as pipes, pipe bundles, finned walls, etc., in the average temperature range cannot be avoided. The dust components bake into stone-like coatings on the components of steel or steel alloys, which on the one hand obstructs heat transfer and on the other hand very strongly promotes corrosion, since HCl and other substances contained in the flue gas are also contained in the coatings and there lead to chloride corrosion of the steel. Apart from the chemical attack by corrosion, there is also, however, a mechanical attack by abrasion through the dust particles contained in the flue gas, wherein abrasive wear can also come about with, for example, the threat of rust by the combustion material itself.
Components such as pipes or pipe bundles therefore have to be regularly demounted and replaced, due to abrasive or corrosive wear, in boiler areas in power stations or refuse incineration plants, which generally means temporary stoppage of the entire plant and which causes annual costs for each plant in the order of magnitude of several hundreds of thousands of Deutsch Marks, in conjunction with an uncoordinated availability. It has already been attempted to deal with this problem by the use of other materials such as, for example, new forms of nickel alloys; thus, EP 0 819 775 describes a nickel-chromium-iron alloy with a reduced content of niobium, which shall have good corrosion resistance and workability. There is known from EP 0 928 658 an Nixe2x80x94Crxe2x80x94Moxe2x80x94Fe alloy which can be welded to a metal support. This alloy, too, shall have a higher corrosion resistance than the usual alloys. A further Nixe2x80x94Crxe2x80x94Co alloy with increased corrosion resistance is described in PCT application WO 97/43457. For the repair of metal pipes with wear locations, EP 0 729 522 describes application, by electroplating, to the inner wall of such a pipe of commercially available alloys based on, for example, nickel, chromium, copper or iron. Wear locations in pipes with a typical diameter between 10 to 50 mm can be repaired in this manner, wherein the repair locations can be approximately 5 to 900 mm long. In addition, the repair alloy applied by electroplating must satisfy specific conditions with respect to layer thickness and compound crystal limits. Moreover, there are known from PCT application WO 98/09751 pipes for incineration plants which consist of an inner pipe component of specific steel alloys and an outer component of a special Nixe2x80x94Crxe2x80x94Moxe2x80x94Nbxe2x80x94Fe alloy, wherein the two components are combined by way of a metallurgical bond, particularly by the Osprey method.
The special alloys proposed in the publications are comparatively expensive, so that they are hardly used in practice. Only the resurfacing weld method also described in, for example, EP 0 729 522, which is also termed xe2x80x9ccladdingxe2x80x9d, has found some applications in practice, but it has proved that the service lives can be improved in the hoped-for degree only to a limited extent, and moreover a welding of elbows and straight pipes is required.
There therefore still exists a need for components for the boiler area of power stations or refuse incineration plants and the like which have a significantly higher resistance against corrosive or abrasive wear and can be produced in a production process without additional weld seams.
For solution of the task there are proposed components for the boiler area of power stations or refuse incineration plants, particularly pipes and pipe bundles, made of steel or steel alloys, which are characterized by a continuous outer electroplated layer of nickel or nickel alloys with a layer thickness of approximately 1 to 5 mm, preferably 1 to 2.5 mm.
Quite unexpectedly it has now been established that a continuous external electroplated layer of pure nickel or nickel alloys, particularly Nixe2x80x94Co alloys, with a relatively small layer thickness of approximately 1 to 2.5 mm suffices to multiply the service lives of such components. Such a nickel coating applied by electroplating obviously prevents not only abrasive wear in all temperature ranges, but quite unexpectedly the deposition of airborne particles of dust on the components and consequent chloride corrosion are thereby largely prevented in the average temperature range. Tests in a refuse incineration plant have shown that the rate of erosion in the case of pipes treated in accordance with the invention was virtually equivalent to zero by comparison with unprotected pipes, which in the same period of use at the same location exhibit an erosion of up to 1.5 mm within a quarter of a year. Since hardly any dust particles had settled on the surface of the pipes, a significantly reduced expenditure on cleaning can also be expected in the future.
Nickel-plating belongs to the oldest method of electroplating technology and has been used in different variants for a long time. The component of steel or steel alloys to be electroplated is in that case connected as a cathode; electrolyte nickel is not usually used as the anode material for nickel-plating, since due to passivation this would dissolve only in certain electrolytes. Processing is normally with a mixture of nickel sulfate and small quantities of nickel chloride as well as boric acid or citric acid. The methods used are state of the art and known to one skilled in the art.
The outer coating applied by electroplating can comprise pure nickel or alsoxe2x80x94according to the respective temperature requirement and other environmental requirementsxe2x80x94nickel alloys; where, for example, there is a threat of rust, the use of an Nixe2x80x94Co alloy is preferred due to its greater wear resistance. Similarly, Nixe2x80x94Mo, Nixe2x80x94P or alloys with a specific content of rare earth metals can also be used for specific purposes.
The nickel-plating can be undertaken not only on pipes and pipe bundles, but also on other plant components such as serpentine heating pipes, finned walls, diaphragm walls, etc.
Nickel-plating is a method enabling a largely stress-free coating. It is even possible, if required, to connect plant components by welding without in that case damaging the nickel-plating.
The particular advantage of the invention resides above all in the fact that the service lives of the loaded components in the boiler area can be multiplied and thereby the otherwise inevitable shutdowns are similarly significantly reduced and standstill periods can be planned. A direct statement still cannot be made about the level of cost reduction such as in the case of a 320,000 ton plant like, for example, the Rugenberger Dam with two boilers (each boiler 160,000 tons), since empirically established values for that are lacking.
However, it can be assumed that the increased cost due to nickel-plating would already be amortized after two years. A service life improvement of at least 8 to 10 years is calculated.